knife bootstrap should be able to create hint files and receive notification to do so from plugins

OHAI-267 created the ability to send hints to Ohai plugins by creating a file in /etc/chef/ohai/hints/ which is the default value for the Ohai hints_path configuration value. For example, to tell the EC2 plugin to assume it is on EC2, you create an empty '/etc/chef/ohai/hints/ec2.json'. By design, these files can take additional JSON attributes but that functionality is not used to date.

knife bootstrap should have a method to take a hint name and an optional hash and create this file.

Other plugins, such as knife-ec2 (KNIFE_EC2-59), should then be able to call this method to ensure that all instances created will have the appropriate hint file.

There is currently a patch for knife-rackspace to do this using the Rackspace API: KNIFE_RACKSPACE-32